Using an interferometer for characterizing a spectral distribution of a radiation is commonplace. The interferometer produces an interference state between two secondary beams that originate from a single initial radiation beam and follow separate intermediate optical paths. A difference in length between the optical paths is varied, causing in turn a variation in the intensity of an output beam formed by superimposing both secondary beams. The spectral distribution of the radiation is obtained based on a Fourier transform of the intensity of the output beam, with respect to a parameter representing the difference in length between the optical paths.
It is thus known that a limited variation range for the difference in length of the optical paths generates a convolution profile in the spectral distribution that results from the Fourier transform. In particular, this convolution profile can have secondary maxima on each side of the main maximum.
It is also known that the difference in length between the optical paths of the two arms of the interferometer depends on the value for the angle of incidence of the radiation. For this reason, an angular aperture that is non-zero for the radiation beams that propagate in the interferometer produces at any moment several interference states, each corresponding to a different value for the difference in length of the optical paths. This results in a modification of the convolution profile in the spectral distribution resulting from the Fourier transform, known by the term self-apodization.
As stated in the article entitled “Communications à la Société Française de Physique”, by P. Connes et al, Journal de Physique, vol 24, February 1963, pages 134-138, the Mertz interferometer allows this self-apodization to be compensated. To this end a component, the optical thickness of which is variable, is introduced into at least one of the two optical paths of the interferometer, and its optical thickness is varied at the same time as the difference in length between the two optical paths is also varied. When the two variations are suitably adjusted in relation to each other, the self-apodization of an interferogram obtained by means of the interferometer is compensated.
FIG. 1a is a functional optical diagram of such an interferometer with self-apodization compensation. The overall structure is that of a Michelson interferometer, with the references indicated in the figure having the following meanings:                F0 and FS initial beam and output beam, respectively,        1 and 7 splitting and compensating plates, respectively,        F1 and F2 secondary beams,        21 and 31 prisms arranged in opposite directions, preferably having equal apex angles and being possibly constituted by one same transparent material,        41 external face of the prism 31, having a reflecting surface effective for the radiation,        6 planor mirror, and        5 compensation assembly, possibly being a transparent plate with parallel faces.        
At least one of the two prisms 21 and 31 is mobile in order to constitute, with the other prism, the variable-thickness optical element. In particular, the two prisms 21 and 31 can be moved simultaneously in opposite directions according to either one of the pairs of black or white arrows shown in the figure, in which an arrow shown on one of the prisms relates to this prism. The self-apodization compensation is obtained close to the optical axis when the following relationship is satisfied for each position of the prisms 21 and 31: D=e·(n−1)/n, where D is the difference in geometrical length between the two optical paths, measured along the axis of the interferometer, e is the difference in thickness between the compensation assembly 5 and the optical element formed by the two prisms 21 and 31, also measured along the axis of the interferometer, and n is the refractive index of the prisms 21 and 31 and of the plate that forms the compensation assembly 5, in the case where these three components are constituted by one same transparent material.
Many applications need to characterize the spectral distribution of a radiation under specific operating conditions. Such is the case, in particular, for analyzing from a satellite of a radiation originating from the Earth's surface. The interferometer must thus be robust, have reduced weight and dimensions, and its operation must be reliable and accurate.